Oncogene jun encodes a sequence-specific trans-activator similar to AP-1

Structure, chromosome location, and expression of the mouse zinc finger gene Krox-20: multiple gene products and coregulation with the proto-oncogene c-fos

We have analyzed the structure and the regulation of Krox-20, a mouse zinc finger-encoding gene which is transiently activated following serum stimulation of quiescent fibroblast cells in culture. The gene is localized on chromosome 10, band B5, in the mouse, and the homologous human gene also maps to chromosome 10 (region q21.1 to q22.1). Alternative splicing of the 5'-most intron of the Krox-20 gene gives rise to mRNAs encoding putative zinc finger proteins with different N termini. The first exon contains a sequence element with strong similarity to the c-fos proto-oncogene serum response element (SRE). This element can functionally substitute for the c-fos SRE, and it binds the same nuclear protein. It is probably responsible for the serum induction of Krox-20, possibly in combination with a weaker SRE located in the 5'-flanking region of the gene. Our findings suggest that c-fos, Krox-20, and a number of immediate-early serum response genes are coregulated and that the SRE and its cognate protein are essential components of this regulatory pathway.

SPXX, a frequent sequence motif in gene regulatory proteins

A new DNA-binding unit, composed of four amino acid residues and common in gene regulatory proteins, is proposed. The occurrences of the sequences Ser-Pro-X-X (SPXX) and Thr-Pro-X-X (TPXX) in gene regulatory proteins are compared with those in general proteins. These sequences are found more frequently in gene regulatory proteins including homoeotic gene products, segmentation gene products, steroid hormone receptors and certain oncogene products, than they are in DNA-binding proteins that are not directly involved in gene regulation, such as the core histones, or in general proteins. It is therefore suggested that these sequences contribute to DNA-binding in a manner important for gene regulation. Amino acid residues characteristic of the types of proteins are found as the variable residues X: basic residues, Lys and Arg, in histones, H1 and sea urchin spermatogenous H2B; Tyr in RNA polymerase II; and Ser, Thr, Ala, Leu and Pro in other gene regulatory proteins S(T)PXX sequences are located on either side of other DNA-recognizing units such as Zn fingers, helix-turn-helices, and cores of histones. The structure of a S(T)PXX sequence is presumed to be a beta-turn I stabilized by two hydrogen bonds, and its potential mode of DNA-binding is discussed.

AP1/jun function is differentially induced in promotion-sensitive and resistant JB6 cells

Tumor promoters may bring about events that lead to neoplastic transformation by inducing specific promotion-relevant effector genes. Functional activation of the transacting transcription factor AP-1 by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) may play an essential role in this process. Clonal genetic variants of mouse epidermal JB6 cells that are genetically susceptible (P+) or resistant (P-) to promotion of transformation by TPA were transfected with 3XTRE-CAT, a construct that has AP-1 cis-enhancer sequences attached to a reporter gene encoding chloramphenicol acetyltransferase (CAT). Transfected JB6 P+, but not P- variants, showed TPA-inducible CAT synthesis. Epidermal growth factor, another transformation promoter in JB6 cells, also caused P+ specific induction of CAT gene expression. These results demonstrate an association between induced AP-1 function and sensitivity to promotion of neoplastic transformation.

Increases in levels of procollagenase messenger RNA in cultured fibroblasts induced by human recombinant interleukin 1 beta or serum follow c-jun expression and are dependent on new protein synthesis

The protein encoded by the protooncogene c-jun, included in the activator protein-1 (AP-1) complex, is probably the critical trans-acting factor controlling transcription of the procollagenase gene which is rate limiting for subsequent synthesis of procollagenase. Therefore, to elucidate possible mechanisms whereby IL-1 stimulates procollagenase synthesis, we measured levels of c-jun and procollagenase mRNA in human serum-starved dermal fibroblasts in response to human recombinant IL-1 beta (hrIL-1 beta). hrIL-1 beta or serum induced rapid increases in c-jun mRNA levels; mRNA levels declined rapidly after hrIL-1 beta and more slowly after exposure to serum. The increases in levels of c-jun mRNA preceded the increases in procollagenase mRNA. Whereas the increases in levels of procollagenase mRNA were blunted by cycloheximide, those of c-jun mRNA were enhanced. We interpret these results as follows: IL-1 or serum induce transcription of c-jun by mechanisms independent of new protein synthesis; c-JUN, the protein product of c-jun in the AP-1 complex, is an essential mediator of the effects of IL-1 or serum in the subsequent induction of expression of the procollagenase gene.

Expression of raf oncogenes activates the PEA1 transcription factor motif

PEA1 (AP1) motif transcription enhancer activity was stimulated by v-raf and more efficiently by activated c-raf-1 or A-raf than by their normal counterparts, in agreement with a role for PEA1 in transformation by raf. Mutations in the ATP-binding site of v-raf prevented activation, suggesting that phosphorylation is somehow required.

Elevated c-fos expression inhibits differentiation of L6 rat myoblasts

Expression of c-fos is induced by a number of signals in several cell systems. Although the exact function of the c-fos product is unknown, it has been implicated to be of importance for both cell growth and differentiation (Verma and Sassone-Corsi, 1987). To analyze how c-fos expression relates to in vitro myogenic differentiation, the kinetics of c-fos mRNA expression during spontaneous in vitro differentiation of L6J1 myoblasts was examined; c-fos transcripts were most abundant at day 4 of the differentiation process. Multinucleated myotubes and expression of alpha-actin and myosin heavy chain (MHC) mRNA appeared later, at day 6 or 7, and increased to maximal levels after 10 days in culture. To analyze further the relation between c-fos expression and L6J1 myogenic differentiation, L6J1 myoblasts were transfected with expression vectors containing the murine c-fos gene driven by a metallothionein promoter. The growth rate of c-fos-transfected L6J1 cells did not differ from that of control cells. However, formation of myotubes was significantly reduced in c-fos-transfected L6J1 cultures compared with neo-transfected controls. Myotube formation and expression of the myogenic markers alpha-actin and MHC were reduced in subclones expressing high levels of c-fos, but not in subclones with lower levels of c-fos expression. These results indicate that a marked elevation of c-fos expression at least partially inhibits L6J1 myogenic differentiation.

The oncogene jun and nuclear signalling

Jun is a transcription factor that can also induce oncogenic transformation. Its DNA-binding domain is conserved from yeast to man and shows homology to several other transcriptional regulators. Jun dimerizes with the fos protein through an alpha-helical domain termed the leucine zipper, and the jun-fos heterodimers bind to DNA and regulate transcription of numerous specific unlinked genes.

Characterization of junD: a new member of the jun proto-oncogene family

In an extensive screen of a cDNA library prepared from serum-stimulated mouse NIH 3T3 cells, we identified three distinct jun-related clones. Two of them were carrying c-jun and junB sequences respectively, whereas the sequence of the third group of clones (junD) was distinct from these two and from v-jun. The amino acid sequences derived from these jun-related clones are very well conserved in five distinct regions including the putative DNA binding domain. Truncated c-Jun and JunD proteins containing the C-terminus recognize the same DNA sequences which were defined as the PEA1/AP1 binding sequence or TPA response element (TRE). Furthermore, both can trans-activate a promoter including the TRE, and this activation is further enhanced by c-fos. Contrary to c-jun and junB transcription, which are strongly stimulated by serum or TPA treatment of quiescent 3T3 cells, junD transcription is not significantly stimulated in these conditions. The tissue distribution and levels of expression of junD mRNA differ from that of c-jun and junB mRNA. These observations suggest that each of these Jun-related gene products has a distinct role in the control of gene activity and growth in the organism.

Yeast GCN4 transcriptional activator protein interacts with RNA polymerase II in vitro

Regulated transcription by eukaryotic RNA polymerase II (Pol II) requires the functional interaction of multiple protein factors, some of which presumably interact directly with the polymerase. One such factor, the yeast GCN4 activator protein, binds to the upstream promoter elements of many amino acid biosynthetic genes and induces their transcription. Through the use of affinity chromatography involving GCN4- or Pol II-Sepharose columns, we show that GCN4 interacts specifically with Pol II in vitro. Purified Pol II is retained on the GCN4-Sepharose column under conditions in which the vast majority of proteins flow through. Moreover, Pol II can be selectively isolated from more complex mixtures of proteins. Conversely, GCN4 protein, synthesized in vitro or in Escherichia coli, specifically binds to the Pol II-Sepharose column under equivalent conditions. Using deletion mutants, we also show that the DNA-binding domain of GCN4 is both necessary and sufficient for this interaction. We suggest the possibility that this GCN4-Pol II interaction may be important for transcription in vivo.

Distinct factors bind the AP-1 consensus sites in gibbon ape leukemia virus and simian virus 40 enhancers

We have demonstrated that the gibbon ape leukemia virus (GALV) enhancer AP-1 element and the simian virus 40 AP-1 enhancer element bind different factors in HeLa nuclear extracts. A 39-kilodalton HeLa nuclear protein and the c-fos protein bind to the GALV element. Antibodies to c-fos abolish binding to the GALV AP-1 site. In contrast, anti-c-fos immunoglobulin fails to inhibit formation of the simian virus 40-specific complex from extracts of HeLa cells. Thus, AP-1-binding complexes are subject to compositional variation at different binding sites.

'Nuclear' oncogenes convert extracellular stimuli into changes in the genetic program

Recent discoveries have advanced our understanding of oncogene action: transcription factors are encoded by cellular (nuclear) oncogenes and the activity of transcription factors is modulated in response to extracellular stimuli. Moreover, the nuclear oncoproteins appear to represent the gene-proximal ends of a network of cellular oncoproteins that link extracellular stimuli to the transcriptional machinery and thus to the program of genes a cell will express.

Multiple genes are transcribed in Hordeum vulgare and Zea mays that carry the DNA binding domain of the myb oncoproteins

cDNA clones were isolated from tissue specific cDNA libraries of barley and maize using as a probe the cDNA of the maize gene C1, a regulator of anthocyanin gene expression. C1-related homology for all of the four cDNAs characterized by sequence analysis is restricted to the N-terminal 120 amino acids of the putative proteins. This region shows striking homology to the N-proximal domain of the myb oncoproteins from vertebrates and invertebrates. Within the myb proto-oncogene family this part of the respective gene products functions as a DNA binding domain. Acidic domains are present in the C-proximal protein segments. Conservation of these sequences, together with the genetically defined regulator function of the C1 gene product, suggest that myb-related plant genes code for trans-acting factors which regulate gene expression in a given biosynthetic pathway.

Forskolin inducibility and tissue-specific expression of the fibronectin promoter

The mechanism of cyclic AMP (cAMP) induction of fibronectin (FN) in HT-1080 and JEG-3 cells differs (D. C. Dean, R. F. Newby, and S. Bourgeois, J. Cell Biol. 106:2159-2170, 1988). In the fibrosarcoma cell line HT-1080, induction requires both protein synthesis and a lag period of 12 to 24 h. In the choriocarcinoma cell line JEG-3, protein synthesis is not required and induction peaks before 24 h, declining thereafter. We show that the FN promoter is transcribed in vitro and that the transcripts initiate at the proper site. Based on transfection experiments with these cells and FN promoter constructions, a cAMP-responsive element (CRE) was identified between -157 and -188 base pairs upstream of the human FN gene. This sequence also conferred cAMP inducibility in both cell lines on the herpesvirus thymidine kinase promoter when it was placed upstream of a thymidine kinase-chloramphenicol acetyltransferase fusion gene. DNase I protection analysis and gel retardation experiments revealed that the CRE was bound by a protein(s) that was present in both HT-1080 and JEG-3 cells as well as in NIH 3T3 cells. Multiple protein-CRE complexes were resolved by gel retardation with extracts of both cell lines. Forskolin treatment of these cells did not alter qualitatively or quantitatively the pattern of CRE-binding proteins that was observed. The FN promoter was at least 10 times more active in HT-1080 than in JEG-3 cells, even though in JEG-3 cells both the rate of FN biosynthesis and the level of accumulated FN mRNA were greater than those in HT-1080 cells. The difference in promoter activity in HT-1080 and JEG-3 cell was mediated by sequences that were located between positions -510 and -56. Deletion of the FN promoter from positions -510 to -56 resulted in an ~30-fold decrease in promoter activity when this construction was transfected into HT-1080 cells, and similar results were observed in NIH 3T3 cells; however, less than a 2-fold effect was observed in JEG-3 cells. Results of these studies suggest that there is some degree of tissue specificity of FN gene expression and reveal that cAMP induction is mediated, in part, by the same element (CRE) in both HT-1080 and JEG-3 cells.

Deregulated expression of human c-jun transforms primary rat embryo cells in cooperation with an activated c-Ha-ras gene and transforms rat-1a cells as a single gene

While the ability of the retroviral oncogene V-jun to transform chicken cells led to its discovery, the oncogenic potential of its cellular homologue, c-jun, which encodes a transcription factor, is unknown. We isolated a 1070-base-pair cDNA clone containing the unmutated entire open reading frame of c-jun from a human small cell lung cancer line. This cDNA as well as a 5.6-kilobase normal human genomic DNA fragment containing the c-jun gene were placed under the control of retroviral long terminal repeats and introduced into primary rat embryo cells (RECs), with or without other oncogenes, and into an immortal rat fibroblast cell line, Rat-1a, as a single gene. In Rat-1a cells the expression of human c-jun mRNA was associated with the ability to clone in soft agarose and form tumors in nude mice. When the c-jun cDNA or genomic DNA constructs were introduced into RECs, no foci of transformed cells were seen with c-jun alone or c-jun cotransfected with deregulated c-myc or L-myc protooncogenes. However, cotransfection of the c-jun constructs with an activated human c-Ha-ras gene led to foci of transformed cells which gave rise to immortalized cell lines that cloned in soft agarose and formed tumors in nude mice. Furthermore, formation of foci of transformed RECs by the c-jun/ras combination was augmented 3-fold by the tumor promoter phorbol 12-tetradecanoate 13-acetate. We conclude that deregulated expression of human c-jun can participate in malignant transformation of normal mammalian cells.

Leucine repeats and an adjacent DNA binding domain mediate the formation of functional cFos-cJun heterodimers

The discovery that the AP-1 family of enhancer binding factors includes a complex of the cellular Fos (cFos) and cellular Jun (cJun) proteins established a direct and important link between oncogenesis and transcriptional regulation. Homodimeric cJun protein synthesized in vitro is capable of binding selectively to AP-1 recognition sites, whereas the cFos polypeptide is not. When cotranslated, the cFos and cJun proteins can form a stable, heterodimeric complex with the DNA binding properties of AP-1/cJun. The related proteins Jun B and vJun are also able to form DNA binding complexes with cFos. Directed mutagenesis of the cFos protein reveals that a leucine repeat structure is required for binding to cJun, in a manner consistent with the proposed function of the "leucine zipper." A novel domain adjacent to, but distinct from, the leucine repeat of cFos is required for DNA binding by cFos-cJun heterodimers. Thus experimental evidence is presented that leucine repeats can mediate complex formation between heterologous proteins and that promotes further understanding of the molecular mechanisms underlying the function of two proto-oncogene products.

Adenovirus transcriptional regulatory regions are conserved in mammalian cells and Saccharomyces cerevisiae

The adenovirus early region 3 (E3) promoter is an early viral promoter which is strongly induced by the adenovirus transactivator protein E1A. DNase I footprinting with HeLa cell extracts has identified four factor-binding domains which appear to be involved in basal and E1A-induced transcriptional regulation. These binding domains may bind TATA region-binding factors (site I), the CREB/ATF protein (site II), the AP-1 protein (site III), and nuclear factor I/CTF (site IV). Recently, it has been shown that the DNA-binding domain of transcription factor AP-1 has homology with the yeast transcription factor GCN4 and that the yeast transactivator protein GAL4 is able to stimulate transcription in HeLa cells from promoters containing GAL4-binding sites. These results suggest an evolutionary conservation of both transcription factors and the mechanisms responsible for transcriptional activation in Saccharomyces cerevisiae and higher eucaryotic organisms. To determine whether similar patterns of transcriptional regulation were seen with the E3 promoter in HeLa and yeast cells, the E3 promoter fused to the chloramphenicol acetyltransferase (cat) gene was cloned into a high-copy-number plasmid and stably introduced into yeast cells. S1 analysis revealed that similar E3 promoter mRNA start sites were found in yeast and HeLa cells. DNase I footprinting with partially purified yeast extracts revealed that four regions of the E3 promoter were protected. Several of these regions were similar to binding sites determined by using HeLa cell extracts. Oligonucleotide mutagenesis of these binding domains indicated their importance in the transcriptional regulation of the E3 promoter in yeast cells. These results suggest that similar cellular transcription factor-binding sites may be involved in the regulation of promoters in both yeast and mammalian cells.

The product of a novel growth factor activated gene, fos B, interacts with JUN proteins enhancing their DNA binding activity

We have identified a gene, fos B, encoding a nuclear protein of 338 amino acids presenting a 70% homology with c-fos, whose expression is activated during G0/G1 transition. Growth factor stimulation of quiescent cells leads to a rapid and transient accumulation of fos B mRNA, with kinetics similar to those of c-fos. The induction of fos B mRNA levels is in part due to a dramatic increase in the transcription of the gene. The half-life of fos B mRNA is in the order of 10-15 min. Both transcriptional activation and mRNA stability are substantially increased in the presence of protein synthesis inhibitors. Immunoprecipitation studies showed that fos B as c-fos protein, forms a complex in vitro with c-jun and jun B proteins in the absence of a target binding sequence. Gel retardation assays demonstrated that fos B protein positively influences the binding of c-jun and jun B proteins to an AP-1 binding consensus sequence, suggesting that fos B protein plays a role in control of gene expression.

Yeast YAP1 encodes a novel form of the jun family of transcriptional activator proteins

The jun family of transcriptional activators includes mammalian AP-1 as well as the yeast regulatory protein GCN4. Recently, an additional transcriptional activator has been found in yeast that recognizes the TGACTCA sequence element common in GCN4/AP-1 sites. This factor was designated yAP-1. The structural gene for yAP-1 has now been isolated and characterized. The deduced amino acid sequence predicts a protein of 650 residues, considerably larger than GCN4 or c-Jun. The amino terminus of yAP-1 is homologous to the carboxy-terminal DNA-binding domains of GCN4 and c-Jun. Disruption of the YAP1 gene demonstrates this gene is not essential but is required for AP-1 recognition element-dependent transcriptional activation. DNA-affinity blots of proteins from YAP1 cells suggest the presence of additional TGACTCA-binding proteins other than GCN4 and yAP-1. Furthermore, expression of at least one of these related DNA-binding proteins appears to be under control of yAP-1.

Dynamic interactions of c-fos protein in serum-stimulated 3T3 cells

The c-fos gene, the cellular homologue of the transforming gene of the FBJ osteosarcoma virus, v-fos, is strongly induced in quiescent BALB/c 3T3 cells by growth factors and in other cell types by a wide variety of transmembrane signalling agents. c-fos is a member of a family of structurally related proteins which includes the fos-related antigens (fra). We have studied the dynamic state of the c-fos protein with an antibody prepared by immunizing rabbits with a plasmid-encoded fos fusion protein. In serum-stimulated BALB/c 3T3 cells, the antibody recognizes a nuclear antigen which resolves on SDS-PAGE as a 60-68-kD group of bands corresponding to c-fos, a doublet at 44-45-kD corresponding to the noncovalently associated p39 protein, as well as an approximately 50-kD band corresponding to a fra. We show that although c-fos protein synthesis is only transiently induced by serum, the c-fos protein persists within the cell after its synthesis has ceased, and it decays with a half-life of 2 hours. Significantly, newly synthesized p39 continues to appear in the immune-precipitated complex even at times when c-fos is no longer synthesized. These kinetics indicate that even following shutoff of c-fos protein synthesis, p39 is newly synthesized and can complex with c-fos protein or a fos-related antigen. During this time, c-fos also undergoes an extensive posttranslational modification. The modification is partially reversed by phosphatase treatment, which implicates protein phosphorylation. Together these results suggest that both interaction with p39 and phosphorylation may progressively modify the properties of c-fos and/or the fos-related antigens over a period of 4-8 hours following the shutoff of fos synthesis. We discuss the implications of the dynamic state of c-fos and fra protein interactions for the function of these proteins.

Growth factors and membrane depolarization activate distinct programs of early response gene expression: dissociation of fos and jun induction

A set of early response genes has been identified whose transcription in fibroblasts is rapidly induced in response to growth factors. Prototype members of this group, c-fos and c-jun, encode products that form a heterodimer and have been implicated in the regulation of gene expression and cell growth. It is thought that other early response genes also encode critical mediators of the cell's response to external stimuli. We have used PC12 pheochromocytoma cells as a model system to test the hypothesis that different extracellular signals induce distinct patterns of expression of early response genes. Our results indicate that membrane depolarization, induced either by potassium chloride or by the neurotransmitter analog nicotine, activates a program of gene expression distinct from that activated by nerve growth factor or epidermal growth factor. Notably, c-fos and c-jun activation can be dissociated; whereas c-jun is coinduced with c-fos and jun-B after growth factor stimulation, membrane depolarization activates c-fos and jun-B without stimulating c-jun. Fos may therefore form transcription complexes with alternative cofactors under different stimulation conditions. nur/77 and zif/268, which encode putative transcription factors, also show markedly different responses to growth factors and depolarization. We conclude that multiple nonconvergent signal transduction pathways control early response gene expression. Our findings also indicate that the diversity and specificity of cellular response to environmental change can be accounted for by the differential combinatorial induction of a relatively small number of early response genes.

Cloning of TPA-inducible early (TIE) genes by differential hybridization using TPA-nonresponsive variant of mouse 3T3-L1 cells

The tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) induces DNA synthesis in quiescent 3T3-L1 cells but not in its variant VT-1 cells. A lambda gt10 cDNA library was constructed using poly(A)+ RNA from 3T3-L1 cells that were stimulated by TPA for 20 min. Radioactive cDNA probes were prepared from mRNAs of TPA-treated 3T3-L1 and VT-1 cells and used for screening of the 3T3-L1 cDNA library by differential hybridization. Nine of 6000 phase plaques hybridized only to the 3T3-L1 cDNA probe. Analysis of the nucleotide sequence of five of these clones indicated a high degree of homology with human or mouse type I and type III collagen genes. Three other independent clones showed no homology with any known DNA sequences. These isolated clones of TPA-inducible early (TIE) genes may be useful to study the signal transduction pathway of phorbol esters.

jun-D: a third member of the jun gene family

The protooncogene c-jun encodes a component of the transcription factor AP-1. Both murine c-jun and a related gene (jun-B) are rapidly activated in BALB/c3T3 cells by serum growth factors. We report here the cloning and analysis of a cDNA encoding a third member of the murine jun family, jun-D. The amino acid sequence encoded by jun-D has two extensive regions of homology with the other Jun proteins. One homology region includes the DNA-binding domain and sequences required for dimer formation and interaction with the Fos oncoprotein; the other includes the acidic sequence thought to be involved in gene activation. All three jun mRNAs are present in a variety of murine tissues and cell lines. In resting 3T3 cells, jun-D is expressed at a higher level compared to c-jun and jun-B, and its transcription is stimulated only slightly by serum growth factors. Thus, jun-D appears to be regulated differently than c-jun and jun-B.

Enhanced jun gene expression is an early genomic response to transforming growth factor beta stimulation

Transforming growth factor beta (TGF beta) is a multifunctional polypeptide that regulates proliferation, differentiation, and other functions of many cell types. The pathway of TGF beta signal transduction in cells is unknown. We report here that an early effect of TGF beta is an enhancement of the expression of two genes encoding serum- and phorbol ester tumor promoter-regulated transcription factors: the junB gene and the c-jun proto-oncogene, respectively. This stimulation was observed in human lung adenocarcinoma A549 cells which were growth inhibited by TGF beta, AKR-2B mouse embryo fibroblasts which were growth stimulated by TGF beta, and K562 human erythroleukemia cells, which were not appreciably affected in their growth by TGF beta. The increase in jun mRNA occurred with picomolar TGF beta concentrations within 1 h of TGF beta stimulation, reached a peak between 1 and 5 h in different cells, and declined gradually to base-line levels. This mRNA response was followed by a large increase in the biosynthesis of the c-jun protein (AP-1), as shown by metabolic labeling and immunoprecipitation analysis. However, differential and cell type-specific regulation appeared to determine the timing and magnitude of the response of each jun gene in a given cell. In AKR-2B and NIH 3T3 cells, only junB was induced by TGF beta, evidently in a protein synthesis-independent fashion. The junB response to TGF beta was maintained in c-Ha-ras and neu oncogene-transformed cells. Thus, one of the earliest genomic responses to TGF beta may involve nuclear signal transduction and amplification by the junB and c-jun transcription factors in concert with c-fos, which is also induced. The differential activation of the jun genes may explain some of the pleiotropic effects of TGF beta.

The 5' flanking region of the pS2 gene contains a complex enhancer region responsive to oestrogens, epidermal growth factor, a tumour promoter (TPA), the c-Ha-ras oncoprotein and the c-jun protein

Expression of the pS2 gene which is transcriptionally controlled by oestrogens in the breast cancer cell line MCF-7 is oestrogen independent in stomach mucosa. We show here that the level of MCF-7 cell pS2 mRNA can also be increased by the tumour promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). We further demonstrate, using transient transfection assays, that the -428 to -332 5' flanking sequence of the pS2 gene contains DNA enhancer elements responsive to oestrogens, TPA, EGF, the c-Ha-ras oncoprotein and the c-jun protein.

An AP1-binding site in the c-fos gene can mediate induction by epidermal growth factor and 12-O-tetradecanoyl phorbol-13-acetate

We have demonstrated that two sequence elements in the c-fos promoter can mediate the response of the gene to epidermal growth factor and the tumor promoter 12-O-tetradecanoyl phorbol-13-acetate (TPA). The first is the previously described serum response element. The second is a sequence element highly homologous to the consensus binding site for the HeLa cell transcription factor AP1. Although recent reports have shown that fos protein binds to AP1-binding sites through an interaction with AP1 protein and have raised the speculation that fos protein may negatively regulate expression of the c-fos gene via this interaction, we found no role for the AP1 consensus homology in the downregulation of c-fos expression following induction by epidermal growth factor and TPA.

AP-1/jun binding sites mediate serum inducibility of the human vimentin promoter

The vimentin gene is inducible by serum in quiescent Balb/c 3T3 cells, but the molecular mechanism of this induction is unknown. The results presented here represent a first step towards the elucidation of the pathway of events leading from growth factor-receptor interaction to this induction. A series of Bal 31 deletions of the human vimentin promoter are used to show that a sequence residing at -700 is responsible for the serum, and also TPA inducibility of this gene. This sequence is able to confer serum inducibility upon uninducible constructs regardless of its position and orientation, indicating that it is this element alone which is required for this induction. The isolated sequence is a strong enhancer as well. Further deletions and the use of synthetic oligonucleotides demonstrate that a 24-mer containing two AP-1/jun binding sites confer both serum and TPA inducibility upon the human vimentin gene. Gel retardation analysis confirms that this sequence binds an AP-1 -like protein.

A cluster of phosphorylation sites on the cyclic AMP-regulated nuclear factor CREB predicted by its sequence

Cyclic AMP regulates the expression of a number of genes through a conserved promoter element, the CRE1. Moreover, transcriptional induction by cAMP requires the activation of cAMP-dependent protein kinase (protein kinase A). We have previously characterized the cAMP response element binding protein (CREB) in PC12 cells and brain tissue as a nuclear factor, of relative molecular mass 43,000, whose transcriptional efficacy is regulated by protein kinase A phosphorylation. CREB stimulates transcription on binding to the CRE as a dimer. Experiments suggesting that the dimerization and transcriptional efficacy of CREB are each stimulated by phosphorylation at distinct sites prompted us to suggest that CREB is regulated by multiple kinases in vivo. We now report the isolation of a cDNA clone for rat CREB using amino-acid sequence information from purified CREB protein. Sequence analysis of this CREB cDNA predicts a cluster of protein kinase A, protein kinase C and casein kinase II consensus recognition sites near the N terminus of the protein. The proximity of these potential phosphorylation sites to one another indicates that they may interact either positively or negatively to regulate CREB bioactivity.

Prolonged activation of jun and collagenase genes by tumour necrosis factor-alpha

Tumour necrosis factor-alpha (TNF-alpha) is secreted by macrophages in response to inflammation, infection and cancer. Sublethal doses of recombinant TNF-alpha to rats causes cachexia, anaemia and inflammation. TNF-alpha plays a major part in tissue inflammation and remodelling by stimulating production of collagenase. Cellular responses to TNF-alpha are initiated by binding to high-affinity cell surface receptors. TNF-alpha then profoundly affects gene regulation, stimulating the fos, myc, interleukin-1 and interleukin-6 genes and inhibiting the type I collagen gene. Here we demonstrate that TNF-alpha also stimulates collagenase gene transcription; this stimulation is mediated by an element of the gene that is responsive to the transcription factor AP-1, the major component of which (jun/AP-1) is encoded by the jun gene; and that TNF-alpha stimulates prolonged activation of jun gene expression. This prolonged induction of jun contrasts with its transient activation by the phorbol ester TPA and provides a physiological example of the ability of jun/AP-1 to stimulate its own transcription. This may be a key mechanism for mediating at least some of the biological effects of TNF-alpha.

The leucine repeat motif in Fos protein mediates complex formation with Jun/AP-1 and is required for transformation

Cellular and viral Fos proteins form a tight complex with other nuclear proteins, including the transcription factor and proto-oncogene AP-1/Jun. We have mapped the c-Jun binding site in Fos to a region containing regularly spaced leucine residues recently suggested to interdigitate with a similar structure in Jun. Substitution of single or multiple leucine residues or the alteration of leucine phasing by insertion of additional amino acids reduces or abolishes the binding to Jun, while the substitution of other amino acids has no noticeable effect. These results strongly suggest that the formation of a "leucine zipper" mediates the interaction between Fos and Jun. We also show that the differential binding of the various Fos mutants correlates with their potential to trans-activate AP-1-dependent transcription and to induce morphological transformation.

A ras-dependent pathway abolishes activity of a muscle-specific enhancer upstream from the muscle creatine kinase gene

Differentiation of skeletal myoblasts is accompanied by induction of a series of tissue-specific genes whose products are required for the specialized functions of the mature muscle fiber. The program for myogenic differentiation is subject to negative control by several peptide growth factors and by the products of mutationally activated ras oncogenes, which persistently activate intracellular cascades normally triggered by specific growth factors. Previously, we reported that induction of the muscle creatine kinase (mck) gene during myogenesis was dependent on a distal upstream enhancer that cooperated with a proximal promoter to direct high levels of expression in developing muscle cells (E. A. Sternberg, G. Spizz, W. M. Perry, D. Vizard, T. Weil, and E. N. Olson, Mol. Cell. Biol. 8:2896-2909). To investigate the mechanisms whereby ras blocks the induction of muscle-specific genes, we have examined the ability of mck 5' regulatory elements to direct expression of the linked reporter gene for chloramphenicol acetyltransferase (cat) in C2 myoblasts bearing mutant N-ras and H-ras oncogenes. In this paper we report that expression of activated ras alleles abolishes activity of the mck upstream enhancer but does not affect the activity of the mck promoter. The ability of ras to repress the expression of mck-cat fusion genes that have been transfected either transiently or stably into myoblasts suggests that ras may exert its effects on muscle-specific genes through mechanisms independent of chromatin configurations or DNA methylation. These results also suggest that ras blocks establishment of the myogenic phenotype by preventing the accumulation of regulatory factors required for transcriptional induction of muscle-specific genes.

Structure, chromosome location, and expression of the mouse zinc finger gene Krox-20: multiple gene products and coregulation with the proto-oncogene c-fos

We have analyzed the structure and the regulation of Krox-20, a mouse zinc finger-encoding gene which is transiently activated following serum stimulation of quiescent fibroblast cells in culture. The gene is localized on chromosome 10, band B5, in the mouse, and the homologous human gene also maps to chromosome 10 (region q21.1 to q22.1). Alternative splicing of the 5'-most intron of the Krox-20 gene gives rise to mRNAs encoding putative zinc finger proteins with different N termini. The first exon contains a sequence element with strong similarity to the c-fos proto-oncogene serum response element (SRE). This element can functionally substitute for the c-fos SRE, and it binds the same nuclear protein. It is probably responsible for the serum induction of Krox-20, possibly in combination with a weaker SRE located in the 5'-flanking region of the gene. Our findings suggest that c-fos, Krox-20, and a number of immediate-early serum response genes are coregulated and that the SRE and its cognate protein are essential components of this regulatory pathway.

Ubiquitous and lymphocyte-specific factors are involved in the induction of the mouse interleukin 2 gene in T lymphocytes

The immediate upstream region of the mouse interleukin 2 (Il-2) gene harbors a strong transcriptional enhancer. This enhancer contains most, if not all of the sequence elements necessary for the T cell specific induction of the Il-2 gene by the phorbol ester TPA and the plant lectin Concanavalin A. DNase I footprinting studies with fractionated extracts obtained from induced and uninduced E14 T cells revealed numerous recognition sites for potential trans-acting factors. Five of these sites are also recognized by the TPA-activated HeLa cell factors AP-1 and AP-3. Other sites including two TATA-boxes, two purine-rich sequence motifs and two copies of the GGGPuTTTCAA motif are recognized by lymphocyte specific factors. The latter motif is highly conserved between several lymphokine genes and is therefore designated as a T cell element (TCE). In E14 T cells, pentamers of the distal TCEd confer an activity similar to that of the entire Il-2 enhancer, whereas in B and HeLa cells, the TCEd-pentamer is inactive as is the Il-2 enhancer. These data indicate the involvement of the TCEd and its recognition factor(s) in the cell type specific induction of the Il-2 gene during T cell activation.

A liver-specific nuclear protein that binds to the distal promoter element of the rat tyrosine aminotransferase gene

Using a gel retardation assay and exonuclease III footprinting, we have analyzed sequence-specific DNA-binding nuclear factors which interact with the distal promoter element of the rat tyrosine aminotransferase gene. A factor called LspA1, binding to a sequence that resembles the consensus binding site for the transcription factor Ap-1, was shown to be present in adult rat-liver nuclear protein extracts but not in the extracts from embryonic liver or spleen nuclei.

Structure of the human genomic glutathione S-transferase-pi gene

The complete human genomic glutathione-S-transferase-pi gene (GST-pi) was isolated from a lambda Charon 4A bacteriophage library which was screened by hybridization to a human GST-pi cDNA. We have sequenced 4261 bp which include the entire GST-pi gene as well as over 1200 bp of the 5' and 200 bp of the 3' flanking regions. The GST-pi gene has 7 exons and 6 introns contained within approximately 2.8 kilobases. Primer extension experiments identified four possible transcription start points closely spaced between 29 and 33 nucleotides (nt) 5' to the start of translation. Analysis of the GST-pi promoter region revealed 4 putative transcription regulatory motifs; these sequences include a 'TATA' box 29 bp upstream from the major transcription start point (nt position -29), 2 Sp1 recognition sequences (GGGCGG, nt positions -46 to -41 and -56 to -51), and an AP-1 recognition sequence (TGACTCA, nt positions -69 to -63). The first 200 nt 5' to the start point of transcription contain a G + C-rich region (79%). Additionally, an intriguing A + T-rich region was found between nt positions -505 and -413 which contained 17 AAAAT tandem repeats. Comparison of the GST-pi gene with the homologous rate gene, GST-P, disclosed extensive conservation of genomic organization between the two species.

Contingent genetic regulatory events in T lymphocyte activation

Interaction of antigen in the proper histocompatibility context with the T lymphocyte antigen receptor leads to an orderly series of events resulting in morphologic change, proliferation, and the acquisition of immunologic function. In most T lymphocytes two signals are required to initiate this process, one supplied by the antigen receptor and the other by accessory cells or agents that activate protein kinase C. Recently, DNA sequences have been identified that act as response elements for one or the other of the two signals, but do not respond to both signals. The fact that these sequences lie within the control regions of the same genes suggests that signals originating from separate cell membrane receptors are integrated at the level of the responsive gene. The view is put forth that these signals initiate a contingent series of gene activations that bring about proliferation and impart immunologic function.

Trans-activation by the c-myb proto-oncogene

We present evidence that the mouse c-myb proto-oncogene encodes a transcriptional trans-activator. Trans-activation was assayed by cotransfection into CV1 monkey kidney cells of a c-myb cDNA expression plasmid together with a reporter plasmid carrying the chloramphenicol acetyltransferase (CAT) gene under the control of a test promoter and enhancer. Cotransfection with the c-myb cDNA plasmid caused a 20-fold stimulation of transcription from the promoter of the mouse alpha 2(I) collagen gene linked to tandem repeats of the simian virus 40 (SV40) enhancer element. Using different promoters in combination with varying numbers of repeats of the SV40 enhancer element, it was shown that tandem repeats of the SV40 enhancer mediated the c-myb-induced activation of transcription. These results show that the mouse c-myb gene product either is itself or induces, an activator of transcription that recognizes specific sequences in the SV40 enhancer.

Interaction of cellular factors related to the Jun oncoprotein with the promoter of a replication-dependent hamster histone H3.2 gene

The promoter region of a replication-dependent histone H3.2 gene contains a putative DNA binding site for the Jun oncoprotein within a 32-nucleotide regulatory domain. The hamster sequence differs by one nucleotide from the AP-1 consensus sequence found in several promoters responsive to phorbol 12-myristate 13-acetate. We have identified the factors interacting with this region as 42- and 45-kDa proteins by DNA affinity chromatography, immunoblotting, and UV crosslinking. These proteins, which are candidates for conferring high-level expression to the histone promoter, share an antigenic epitope with the DNA-binding domain of Jun but diverge from it at the amino terminus. The interaction of these proteins with the promoter of a replication-dependent cellular gene provides evidence that members of the Jun oncoprotein family may play specific roles in the expression of genes essential for progression of the cell cycle.

Glucocorticoid and cAMP induction mechanisms are differentially affected by the p85gag-mos oncoprotein

The inability to perceive and coordinate both internal and external signals that function to regulate cellular growth and proliferation is a hallmark of oncogenic transformation. To examine the effects of the v-mos oncogene on distinct signal transduction pathways, the 6m2 cell line was used, in which expression of the p85gag-mos oncogene, and consequently transformation, are temperature sensitive. Through the analysis of endogenous metallothionein 1 (Mt-1) gene expression in 6m2 cells, p85gag-mos effects on glucocorticoid, cAMP, and heavy-metal induction were examined. While heavy-metal induction of Mt-1 mRNA was found to be unaffected by p85gag-mos, differential effects were exerted upon glucocorticoid and cAMP induction of Mt-1. Glucocorticoid induction of Mt-1 mRNA in p85gag-mos-transformed 6m2 cells was initiated normally but not maintained to the same extent as in nontransformed 6m2 cells. In contrast, cAMP did not induce Mt-1 mRNA in p85gag-mos-transformed 6m2 cells, although a significant induction was noted in nontransformed 6m2 cells. Thus, an oncoprotein interferes with different steps in each particular signal transduction pathway, ultimately causing abnormalities of inducible gene expression.

Induction of metallothionein mRNA by tumor promoters in mouse skin and its constitutive expression in papillomas

A single topical application of 12-O-tetradecanoylphorbol-13-acetate (TPA) was found to induce mRNA of a metallothionein (MT) gene or genes in the skin of Sencar mice, and papillomas produced by repeated applications of TPA were shown to have elevated levels of MT mRNA. Induction of MT mRNA was maximal 4-8 h after application of TPA and returned to the control level 24 h later. A dose-dependent increase of MT mRNA was observed with doses of TPA of 1-5 micrograms. Of the other promoters tested, phorbol-12, 13-didecanoate, mezerein, and the ionophore A23187 also induced MT mRNA, but 4-O-methyl-TPA and benzoyl peroxide did not. Phorbol and 4 alpha-phorbol-12,13-didecanoate, which are not promoters, also did not induce MT mRNA. Retinoic acid and 1 alpha, 25-dihydroxyvitamin D3, inhibitors of tumor promotion, did not induce MT mRNA themselves or inhibit the induction of MT mRNA by TPA. In C57BL/6 promotion-resistant mice, TPA caused only slight induction of MT mRNA. These data suggest a correlation between induction of MT mRNA and epidermal hyperplasia. The constitutive elevation of MT mRNA levels in papillomas may be due to the loss, during the process of tumor promotion, of some mechanism regulating MT gene expression.

Molecular cloning of the chicken myelomonocytic growth factor (cMGF) reveals relationship to interleukin 6 and granulocyte colony stimulating factor

Normal as well as retrovirally transformed avian myeloid precursor cells require the colony stimulating factor cMGF for their survival, proliferation and colony formation in vitro. cMGF has been shown to be a glycoprotein which is active in the picomolar concentration range. Co-expression of kinase type oncogenes in v-myb or v-myc transformed myeloid cells induces cMGF expression and confers factor independence via an autocrine mechanism. Here we describe the molecular cloning of cMGF from a myeloblast cDNA library and show that it is a 201 amino acid residue secretory protein which is modified by signal peptide cleavage and glycosylation during translocation into the lumen of membrane vesicles. A bacterially expressed trpE-cMGF fusion protein induces proliferation of E26 transformed myeloblasts in a cMGF bioassay suggesting that glycosylation is not absolutely necessary for biological activity. Sequence comparison reveals that cMGF is distantly related to G-CSF and IL-6.

AP1, a composite transcription factor implicated in abnormal growth control

Growth factors activate cascades of intracellular events, some of which result in altered gene expression. A series of recent discoveries have highlighted the role of the transcription factor AP1 as a mediator of the effects of growth factors, as well as those of oncogenes and the tumour promoter TPA. We discuss the molecular composition of AP1, how its activity is thought to be regulated, and the evidence that AP1 activation is involved in transformation.